ScienceDaily (Mar. 1, 2012) — Until now, it was presumed that the last glacial period denuded the Scandinavian landscape of trees until a gradual return of milder weather began and melted away the ice cover some 9000 years ago. That perspective is now disproved by research headed by Professor Eske Willerslev from the Centre for GeoGenetics, University of Copenhagen, Denmark, Laura Parducci from the University of Uppsala, Sweden, and Inger Greve Alsos from Tromsø University Museum, Norway. Their research teams show that some Scandinavian conifers survived the inhospitable ice age climate likely for several thousands of years. The result is to be published in the scientific journal Science.

The story of Scandinavian forests needs revamping when it comes to the history of conifers, spruce and pine in particular. Until now, researchers believed that contemporary coniferous forests in Scandinavia were the products of species migration from the areas of southern and eastern Europe that were ice-free during the last ice age. Indeed, the last glacial period saw Scandinavia covered by a formidable ice sheet.

Ice-free pockets

The migration interpretation of the story is not correct, as the picture of Scandinavian coniferous forests is far more nuanced and complex than previously thought.

"Our results demonstrate that not all the Scandinavian conifer trees have the same recent ancestors, as we once believed. There were groups of spruce and pine that survived the harsh climate in small ice-free pockets, or in refuges, as we call them, for tens of thousands of years, and then were able to spread once the ice retreated. Other spruce and pine trees have their origins in the southern and eastern ice-free areas of Europe. Therefore, one can now refer to 'original' and later naturally 'introduced' Scandinavian conifer species," says Professor Eske Willerslev, Centre for GeoGenetics, University of Copenhagen.

In the midst of an ice age

The spectacular results have emerged, in part, by studying the DNA of modern spruce -- which clearly portray two Scandinavian types -- and also by analysing the composition of pine and spruce DNA in sediments from lake-core samples. Additionally, researchers analysed other ancient DNA and the remains of macrofossils to reach their conclusions.

Two locations in Norway have proved particularly lucrative for the researchers. One of them, Andøya Island, in north-western Norway, is the source of material dated between 17,700 and 22,000 years-old. During the last ice age, the island was an ice-free pocket, one "refuges" on the edge of the enormous ice sheet, which blanketed at that time nearly all of Scandinavia.

"The other evidence, which supports the surviving conifers in the midst of an ice age, originates in Trøndelag, central Norway. One hypothesis is that trees were able to survive on the top of nunataks, the exposed ridges or peaks of mountains protruding from glacial cover, or in more sheltered areas close to the coast where proximity to the temperate conditions of the Atlantic Ocean favoured survival. These areas must have provided sites for roots to anchor and trees to grow in the challenging climate," says Laura Parducci, University of Uppsala.

Today, nunataks can be found protruding from the Greenlandic ice sheet, though without any trees to adorn them.

Money in trees

According to Inger Greve Alsos, Tromsø University Museum, their results are not just useful within the context of revising the history of Scandinavian conifers.

"The essence of our studies is that they challenge conventional scientific notions of the spreading of trees, biodiversity and survival in harsh environments from a global perspective; especially with regards to climate change or other changes and interventions in nature. I also believe that our results will have economic significance. We now know that there are two types of naturally occurring spruce in Scandinavia. These two trees have very different histories and therefore it can be expected that they have differing qualities -- for example in their hardness and the like. Previously, we thought that the differences between naturally occurring spruce in Scandinavia were due to simple individual variations. Now we would like to determine if these differences relate to one or another type of spruce by the use of a simple DNA tests. This will mean a great deal to tree plantation owners and others who would like to grow spruce with particular qualities," explains Inger Greve Alsos.

Interesting that trees may have survived. There are a number of pockets that by chance happened to escape the glaciations, or at least the last ones. The lower edge of the continental ice sheets tended to be more like fingers extending down into valleys much like mountain glaciers, rather than a faceless uniform wall of ice. The direction and reach of these fingers were both dependent on the existing ground topography and the thickness and direction of flow of the main masses of the continental glaciers. A large areas of southwestern Wisconsin (and parts of Minnesota and Iowa) are called the driftless area because they were bypassed by the ice sheets and are not covered by glacial sediment (drift). The topography in these areas is rougher as it has not been smoothed and planed off by the glaciers.

These edges of the ice sheet extended fairly far down into warmer areas. There outer edge was where the rate of melting back equaled the rate of forward flow of the ice, like a giant unidirectional conveyer belt going generally southward. So the region had to be relatively warmer to melt the ice at the fringe as fast as it flowed onward. There would be effects directly caused by the presence of the ice itself - cold winds flowing down the ice sheet, direct cooling by the ice mass, but away from the ice a relatively short distance the overall climate could be much warmer. The stomach contents of some wooly mammoths found preserved in the Siberian tundra are indicative of the warmer weather in the area just beyond the ice sheets. Many of the plants found are not tundra mosses, but plants normally found farther south in today's climate. Still if the areas were surrounded by ice, I would have expected they would have been to cold to support trees. Perhaps these were in areas that large enough to not be completely dominated by the glacier effects, or where at any one time the glaciation affected only one edge of the area at a time and did not completely surround the regions.

Edward Frank

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Ed

"I love science and it pains me to think that so many are terrified of the subject or feel that choosing science means you cannot also choose compassion, or the arts, or be awe by nature. Science is not meant to cure us of mystery, but to reinvent and revigorate it." by Robert M. Sapolsky

By the way, Ed- I wish I had majored in geology, as I think you did- either that, or astronomy- both are awesome sciences. I do like forestry as a practice, it's good for the exercise, but it's a field without much intellectual power.Joe

I guess its not too surprising that spruce would survive in a refugium not covered by ice. All its needs is 6 weeks where daytime temperatures are above 50 degrees F to finish its life cycle. I suppose its possible that black spruce or tamarack may one day be discovered to have persisted in SW Wisconsin during the glaciation--both can grow on permafrost. On one hand, the severity of the interior continental climate may have prevented their persistence (compared to the oceanic climate of western Norway). On the other hand, SW Wisconsin is farther south and solar radiation would have been quite strong relative to northern Norway, so summers might have been warm enough. There aren't many if any bogs or lakes (which are found in the glaciated areas) in the driftless area, but there might be streamside deposits such as cutoff oxbows with fossil evidence from the glacial period.

There has been a lot of post glacial stratigraphy work done in the driftless in terms of river valley deposition/erosion cycles. It is especially "famous" for the massive aggradation that has occur in the past 150 years.